The present disclosure relates to an image sensor.
Image sensors are semiconductor devices that convert an optical image into an electrical signal. The image sensors are generally classified into a charge-coupled device (CCD) image sensor and a complementary metal-oxide-silicon (CMOS) image sensor.
In general, a photodiode of an image sensor is formed through an ion implantation process on a substrate. However, as the size of a photodiode decreases in order to increase the number of pixels without increasing the chip size, the image quality degrades due to a decrease in the area of a light receiving unit.
Also, because the stack height (e.g., the combined thicknesses of the layers of materials on the image sensor) generally does not decrease as much as the corresponding decrease in the area of the light receiving unit, the number of photons entering the light receiving unit also decreases due to a light diffraction phenomenon called an airy disk.
As an alternative solution for overcoming the above limitation, an attempt is being made to form a photodiode by depositing amorphous silicon, or to form a readout circuit on a silicon substrate and form a photodiode on the readout circuit through a wafer-to-wafer bonding process, which is hereinafter referred to as a three-dimensional image sensor. Herein, the photodiode and the readout circuit are connected through a metal line (or interconnection).
In fabricating a three-dimensional image sensor, a wafer bonding process and a cleaving process are performed to form a photodiode layer on the image sensor. A hydrogen ion implantation process is performed for the cleaving process. However, the hydrogen ion implantation may cause a defect in the crystal structure of the wafer surface, and the defect in the wafer surface may degrade the dark characteristics of the image sensor (e.g., a dark current).
Also, a cleaving process is performed to form a photodiode with a desired thickness. However, the photodiode may have a non-uniform surface due to the cleaving process, which may degrade the bonding force (e.g., with either the substrate having subsequently deposited materials).
Meanwhile, because both the source and the drain of the transfer transistor are doped with a high concentration of N-type impurities, the related art approach may cause a charge sharing phenomenon as shown in FIG. 15. The charge sharing phenomenon may degrade the output image sensitivity, which may cause an image error. Also, the related may not smoothly transfer photo charges between the photodiode and the readout circuit, thus generating a dark current or degrading the saturation or sensitivity characteristics.